Titre du document / Document title

Isotopic fractionation of sulfur during formation of basaluminite, alunite, and natroalunite

Auteur(s) / Author(s)

PRIETZEL Jörg ; MAYER Bernhard ;

Résumé / Abstract

The extent of fractionation of sulfur (S) isotopes during precipitation of basaluminite and alunite in supersaturated Al²(SO⁴)³ solutions was determined in a laboratory synthesis experiment. An acid 0.05 M Al²(SO⁴)³solution was partly neutralized with Ca(OH)², NaOH, and KOH to a molar OH/Al ratio of 2.0. Subsequent aging of the precipitates for 15 weeks at temperatures of 20 and 50 °C resulted in the formation of poorly crystalline basaluminite [Al⁴(SO⁴)(OH)¹⁰.4H²O], of mixtures of basaluminite with crystalline natroalunite [NaAl³(SO⁴)²(OH)⁶] or alunite [KAl³(SO⁴)²(OH)⁶], and of pure crystalline alunite. No significant isotopic fractionation of S was observed during the precipitation of poorly crystalline basaluminite (Δδ₃₄S^{basaluminite-solution}=+0.04′0.34‰). In contrast, the formation of alunite and natroalunite was accompanied by small but significant isotopic fractionation that favored the incorporation of the heavier isotope, ₃₄S, in the minerals. Sulfur-isotope fractionation (Δδ₃₄S^{mineral-solution}) was +0.84′0.34%o for alunite, and +1.72+0.34‰ for natroalunite. The extent of S-isotope fractionation was found to be dependent on (i) the chemical composition and (ii) the crystal structure of the synthesized minerals, and seems to be related to the physico-chemical properties of the involved ions (charge, radius, mass), the energy balance of the reaction, and the reaction rate. The results indicate that S-isotope mass balances in aerated acid soils are not significantly confounded by S isotopic fractionation effects associated with the formation of Al hydroxysulfates.

Revue / Journal Title

Chemical geology   ISSN 0009-2541   CODEN CHGEAD 

Source / Source

2005, vol. 215, n^o1-4, pp. 525-535 [11 page(s) (article)]

Langue / Language

Anglais

Editeur / Publisher

Elsevier, Amsterdam, PAYS-BAS  (1966) (Revue)

Localisation / Location

INIST-CNRS, Cote INIST : 12905 A, 35400012934197.0230